Tempered Melt-Blown Nonwoven Having a High Compression Hardness

ABSTRACT

The invention relates to a method for producing a tempered melt-blown nonwoven, comprising the following steps: a) producing a melt-blown nonwoven preferably by applying flowing air to the outside of polymer melt extruded through a nozzle and stretching said polymer melt before the filaments thereby formed are laid and cooled on a carrier, which is preferably a double suction drum, and b) tempering at least one section of the melt-blown nonwoven produced in step a) at a temperature that lies between the glass transition temperature and 0.1° C. below the melt temperature of the filaments of the melt-blown nonwoven, the melt-blown nonwoven having a weight per unit area of 100 to 600 g/m2, a density of 5 to 50 kg/m3 and a compression hardness at 60% compression, measured according to DIN EN ISO 3386, of at least 2 kPa. The invention further relates to a tempered melt-blown nonwoven produced by means of said method, preferably a tempered voluminous melt-blown nonwoven. Said tempered melt-blown nonwoven is characterized by a significantly increased compression hardness in comparison with an untempered melt-blown nonwoven.

The present invention relates to an annealed meltblown nonwoven with a high compression hardness and particularly to an annealed voluminous meltblown nonwoven with a high compression hardness. The present invention further relates to a method for manufacturing such an annealed meltblown nonwoven fabric.

Felts and nonwoven fabrics are usually manufactured from staple fibers and/or continuous filaments using known mechanical or aerodynamic methods. One known aerodynamic process is the meltblown method based on the Exxon principle as described in U.S. Pat. No. 3,755,527, for example. In that method, a low-viscosity polymer is extruded through capillaries located at a nozzle tip. The polymer droplets that form are then acted upon from two sides with a high-temperature, high-speed air flow that is referred to as blast air, whereby the polymer droplets are drawn into a free polymer jet in the form of fine filaments. As a result of the streams of air impinging on the polymer droplets at an acute angle, an oscillation process that is present in the free jet then present is then induced in the free polymer jet, resulting in the occurrence of high-frequency processes that accelerate the polymer strands beyond the speed of the blast air. The polymer strands are thereby additionally stretched, so that the filaments obtained after deposition of the filaments on a carrier and after cooling can have a diameter and a fineness of a few micrometers in the single-digit range or even less. The meltblown nonwoven fabrics or meltblown nonwovens manufactured in this manner are used for a variety of applications, such as barrier functions in the hygiene sector, for example. For these applications, the filaments are deposited on the carrier as a flat, two-dimensional nonwoven fabric.

Another known meltblown process was developed by Biax Fiberfilm Corp. and is described, for example, in U.S. Pat. No. 4,380,570.

Voluminous, three-dimensional meltblown nonwoven fabrics can also be manufactured by depositing the filaments formed between two suction drums or double drums, as described in DE 17 85 712 C3 and in U.S. Pat. No. 4,375,446, for example. These voluminous meltblown nonwovens can be used, for example, as oil absorbers or as acoustic damping materials. However, these voluminous meltblown nonwoven fabrics have the disadvantage that they are highly ductile and characterized by poor relaxation, which results in a loss of volume following a compressive load.

Meltblown nonwoven fabrics are known from U.S. Pat. No. 4,118,531 which, in addition to the meltblown filaments, contain staple fibers incorporated therein that are composed of polyethylene terephthalate. These nonwoven fabrics are characterized by increased rebound elasticity, which is why the nonwoven has better relaxation. However, these nonwoven fabrics are composed of two polymers that are mutually incompatible, which precludes recycling and thus results in a sizable cost disadvantage.

For some nonwoven applications, such as the use thereof as acoustic damping materials, the nonwoven fabrics must be voluminous—i.e., have a large internal void volume. One major drawback of the known voluminous meltblown nonwovens is their comparatively low stiffness and resulting low compression hardness, particularly under greater loads. Furthermore, these materials are usually limp, which means that they deform under their own weight but do not retain any particular shape. For these reasons, it is only with difficulty that a predetermined shape can be imparted permanently to these known meltblown nonwoven fabrics, particularly to these known voluminous meltblown nonwoven fabrics. Deformation also generally results in a compression of these nonwoven fabrics.

It is therefore the object of the present invention to provide a voluminous meltblown nonwoven fabric that has increased stiffness and, in particular, increased compression hardness, particularly under greater loads, and that also retains its thickness-specific acoustic characteristics, such as acoustic absorption coefficient and to which a predetermined permanent shape can also be easily imparted.

According to the invention, this object is achieved by an annealed meltblown nonwoven fabric that can be obtained by means of a method in which at least a portion of the meltblown nonwoven fabric is subsequently annealed at a temperature between the glass transition temperature and 0.1° C. below the current melting temperature of the filaments of the meltblown nonwoven fabric (15), the meltblown nonwoven fabric having a weight per unit area of from 100 to 600 g/m², a density of from 5 to 50 kg/m³, and preferably a compression hardness at 60% compression of at least 2 kPa as measured according to DIN EN ISO 3386.

This solution is based on the surprising insight that a voluminous meltblown nonwoven fabric that is subsequently annealed at a temperature between the glass transition temperature and 0.1° C. below the melting temperature of the filaments of the meltblown nonwoven fabric, namely one with a weight per unit area of from 100 to 600 g/m² and with a density of from 5 to 50 kg/m³, has a significantly increased stiffness compared to the corresponding unannealed meltblown nonwoven fabric. As a result, the voluminous meltblown nonwoven fabric according to the invention is characterized by a significantly increased compression hardness above all under greater loads such as at 40% or 60% compression, for example, namely by a compression hardness at 60% compression of at least 2 kPa. Such high compression hardnesses cannot be achieved for such voluminous meltblown nonwovens without annealing. Furthermore, the voluminous meltblown nonwoven fabric according to the invention can be easily shaped into a desired shape during annealing. Without wishing to be bound to a theory, it is supposed that these benefits are at least partly due to the fact that the degree of crystallinity of the nonwoven filaments, which are predominantly amorphous beforehand, is significantly increased during the annealing that is subsequently performed according to the invention. This is supposed because the inventors have observed that, depending on the conditions during annealing, the melting temperature of the filaments of the meltblown nonwoven fabric can be increased by about 10 to 20° C. as a result of the annealing. The experiments conducted by the inventors appear to show that, due to the very high pay-off speeds during the manufacture of the filaments to very thin finenesses of the filaments, the polymer melt cools rapidly despite the hot air blast, whereby the amorphous molecular structure of the melt is “frozen in place,” as it were. As stated, the degree of crystallinity of the amorphous nonwoven filaments is increased by the annealing according to the invention. Advantageously, the filament fineness and the nonwoven structure are not altered or only insignificantly at most by the annealing, so that the voluminous nonwoven fabric retains its other characteristics after annealing, namely its thickness-specific acoustic characteristics such as its absorption coefficient.

In terms of the present invention, a “meltblown nonwoven fabric” is understood to be a nonwoven fabric that is manufactured using one of the known meltblown methods, independently of whether it is a flat 2-dimensional nonwoven fabric or a voluminous nonwoven fabric. Methods for manufacturing such meltblown nonwoven fabrics are described in U.S. Pat. No. 4,118,531, in U.S. Pat. No. 4,375,446, in U.S. Pat. No. 4,380,570, and in DE 17 85 712 C3, for example.

In addition, for the purposes of the present invention, “annealing” is generally understood as a heat treatment, i.e., the heating of the meltblown nonwoven fabric at the abovementioned temperature for a defined period of time.

According to the invention, at least a portion of the meltblown nonwoven fabric is subsequently annealed, namely at a temperature between the glass transition temperature and 0.1° C. below the melting temperature of the filaments of the meltblown nonwoven fabric. Here, both the glass transition temperature and the melting temperature of the filaments of the meltblown nonwoven fabric refer to the corresponding temperatures of the meltblown nonwoven fabric at that point in time. As set forth above, the inventors have observed that, depending on the conditions during annealing, the melting temperature of the filaments of the meltblown nonwoven fabric can be increased by about 10 to 20° C. as a result of the annealing. The temperature can thus be increased during annealing. For example, if the melting temperature of the filaments of the meltblown nonwoven fabric is 152° C. before annealing is commenced, and the melting temperature of the filaments of the meltblown nonwoven fabric is increased to 170° C., the annealing can be carried out in such a way, for example, that the meltblown nonwoven fabric is initially annealed at a temperature of 150° C., the temperature is increased to 155° C. (which is 2° C. below the melting temperature that the filaments of the meltblown nonwoven fabric have at this point in time) after a defined period of time of 10 minutes, for example, before the temperature is increased to 165° C. (2° C. below the melting temperature that the filaments of the meltblown nonwoven fabric have at this point in time) after another defined period of time of 10 minutes, for example.

The meltblown nonwoven fabric is partially or fully annealed in the process. A certain portion of the meltblown nonwoven fabric or multiple portions of the meltblown nonwoven fabric be annealed, whereas the rest of the meltblown nonwoven fabric remains unannealed. It is also possible and, according to the present invention, also especially preferred for the entire meltblown nonwoven fabric to be annealed.

Good results both in terms of plasticity and in terms of increasing the rigidity and particularly the compression hardness of the annealed meltblown fabric are achieved particularly if the meltblown nonwoven fabric or the subregion(s) thereof to be annealed are annealed at a temperature that lies between 20° C. below the melting temperature and 0.1° C. below the melting temperature of the filaments of the meltblown nonwoven fabric. Especially preferably, the annealing is performed at a temperature that lies between 15° C. below the melting temperature and 1° C. below the melting temperature, more preferably between 10° C. below the melting temperature and 0.1° C. below the melting temperature, very especially preferably between 5° C. below the melting temperature and 0.1° C. below the melting temperature, such as for example about 5° C. below the melting temperature (i.e., between 8° C. below the melting temperature and 2° C. below the melting temperature, for example), and most preferably between 2° C. below the melting temperature and 1° C. below the melting temperature of the filaments of the meltblown nonwoven fabric.

The duration of the annealing depends on the temperature to which the meltblown nonwoven is heated during the annealing, with a lower annealing temperature tending to require a longer annealing time. In principle, an annealing period of from 1 minute to 10 days and particularly from 2 minutes to 24 hours has proven to be suitable. The period of annealing is preferably from 2 minutes to 2 hours, especially preferably from 2 to 60 minutes, and most preferably from 2 to 10 minutes.

Good results are achieved particularly if the meltblown nonwoven fabric is annealed for 2 minutes to 2 hours at a temperature that lies between 20° C. below the melting temperature and 1° C. below the melting temperature of the filaments of the meltblown nonwoven fabric. Especially preferably, the annealing of the meltblown nonwoven fabric is performed for 2 to 60 minutes at a temperature that lies between 15° C. below the melting temperature and 2° C. below the melting temperature of the filaments of the meltblown nonwoven fabric, and very especially preferably, the annealing of the meltblown nonwoven fabric is performed for 2 to 10 minutes at a temperature that lies about 5° C., i.e., between 8° C. below the melting temperature and 2° C. below the melting temperature of the filaments of the meltblown nonwoven fabric.

As stated above, the melting point of the meltblown nonwoven fabric can increase during annealing due to the increase in the degree of crystallinity. At a constant annealing temperature, the interval between the annealing temperature and the melting point of the meltblown nonwoven fabric would increase more and more during annealing in this case, so the required annealing time would be relatively long. Therefore, it is proposed according to an alternative embodiment of the present invention that the temperature be increased during annealing in order to always keep the tempering temperature just below (e.g., about 2° C. or 5° C. below) the melting point of the meltblown nonwoven fabric, which increases during annealing. For example, if the melting temperature of the filaments of the meltblown nonwoven fabric is 152° C. before annealing is commenced, and the melting temperature of the filaments of the meltblown nonwoven fabric is increased to 170° C., the annealing can, as explained above, be carried out in such a way, for example, that the meltblown nonwoven fabric is initially annealed at a temperature of 150° C., the temperature is increased to 155° C. (which is 2° C. below the melting temperature that the filaments of the meltblown nonwoven fabric have at this point in time) after a defined period of time of 10 minutes, for example, before the temperature is increased to 165° C. (2° C. below the melting temperature that the filaments of the meltblown nonwoven fabric have at this point in time) after another defined period of time of 10 minutes, for example.

In principle, the present invention is not restricted in terms of the manner in which the meltblown nonwoven fabric is annealed. In the context of the invention, annealing in which the meltblown nonwoven fabric is exposed to hot air and/or with superheated steam has been found to be not only simple, but also especially effective. In this embodiment, the hot air or superheated steam has a temperature that corresponds to the temperature to which the meltblown nonwoven fabric is to be heated during annealing. In this embodiment, the meltblown nonwoven fabric is preferably exposed to hot air or superheated steam by having the hot air or superheated steam flow around or, more preferably, flow through the meltblown nonwoven fabric.

To achieve this, the meltblown nonwoven fabric is preferably annealed in a furnace having at least one blast box that is arranged such that the hot air or the superheated steam can be blown into the meltblown nonwoven fabric. Insofar as only one or more portions of the meltblown nonwoven fabric are to be annealed, the blast box is to be designed such that the hot air or superheated steam is blown only into the portion(s) of the meltblown nonwoven fabric that are to be annealed. In a development of the inventive idea, it is proposed that the meltblown nonwoven fabric be annealed in a furnace having at least one suction box that is arranged such that air or superheated steam flowing through the meltblown nonwoven fabric can be extracted in order to ensure a reliable flow. The application of a vacuum on both sides ensures that the hot air or the superheated steam flows reliably through the nonwoven fabric and that the nonwoven fabric does not collapse but maintains its volume.

According to an especially preferred embodiment of the present invention, the meltblown nonwoven fabric is annealed in a furnace that has at least one blast box and at least one suction box, with the at least one blast box being arranged such that the hot air or the superheated steam can be blown into the meltblown nonwoven fabric, and with the at least one suction box being arranged such that the air or superheated steam flowing through the meltblown fabric can be extracted. In this embodiment, the furnace especially preferably has two blast boxes and one or two suction boxes; in the case of one suction box, the suction box is arranged downstream from the first or second blast box, and in the case of two suction boxes, both suction boxes are arranged downstream from the first and the second blast box.

According to the invention, the meltblown nonwoven fabric has a weight per unit area of from 100 to 600 g/m². Especially good results are achieved particularly with respect to the acoustic properties of the nonwoven fabric if the weight per unit area of the meltblown nonwoven is from 150 to 400 g/m², particularly preferably 200 to 400 g/m², and very especially preferably 250 to 350 g/m², e.g., about 350 g/m².

In terms of the acoustic properties achieved, it is also preferred that the meltblown nonwoven fabric be a voluminous meltblown nonwoven fabric with a density of from 7 to 40 kg/m³, more preferably 8 to 25 kg/m³, and especially preferably 10 to 20 kg/m³.

In principle, the filaments of the meltblown nonwoven fabric can be composed of any polymer having a melting point that is suitable for extrusion and a sufficiently low viscosity in the molten state for the meltblown process, such as polyolefins, polyamides, polyesters, polyphenylene sulfides, polytetrafluoroethylenes, or a polyether ether ketone, for example. Examples of polyesters are polyethylene terephthalate and polybutylene terephthalate. Filaments made of polyolefin and especially preferably of polypropylene and/or polyethylene have proven to be especially suitable. According to the present invention, the filaments of the meltblown nonwoven fabric are very especially preferably composed of isotactic polypropylene, since it has been found that the degree of crystallinity is increased especially well during annealing in the case of filaments of isotactic polypropylene.

The thickness of the meltblown nonwoven fabric is preferably 6 to 50 mm, more preferably 8 to 40 mm, very especially preferably 10 to 30 mm, and most preferably 15 to 25 mm, such as particularly about 20 mm.

For materials that do not exhibit particularly good crystallization behavior, this can be increased through the addition of crystallization seeds during the extrusion process.

In a development of the inventive idea, it is proposed that the meltblown nonwoven fabric be annealed in a mold body in order to also impart a predefined shape to the meltblown nonwoven fabric during annealing. This can be achieved, for example, by having the mold in which the meltblown nonwoven fabric is annealed be embodied at least partially as a screen, so that the meltblown nonwoven fabric can be flowed through and/or flowed around by hot air or with superheated steam during annealing.

In an alternative embodiment, it is proposed that the meltblown nonwoven fabric be placed into a mold body after heating but before cooling and thereby transformed into a predefined shape, with the meltblown nonwoven fabric being cooled in the mold in order to conclude the annealing process.

This makes it possible, for example, for the meltblown nonwoven fabric to be shaped by the annealing as a stamped part into a specific shape, such as a hemisphere, for example. The meltblown nonwoven fabric that is annealed and shaped in this way has substantially greater dimensional stability than the starting material and retains its shape to the greatest possible extent. The meltblown nonwoven fabric can therefore take on strengths after annealing that enable additional stiffening structural elements in the meltblown nonwoven to be dispensed with after molding.

According to another preferred embodiment of the present invention, a provision is made that at least one spacer is arranged in the meltblown nonwoven fabric in the thickness direction of the meltblown nonwoven fabric that has a length that is greater than the thickness of the meltblown nonwoven fabric. This is advantageous, for example, if the meltblown nonwoven fabric is to be used as an acoustic absorber. Through the molding of the spacer or spacers into the rigid meltblown nonwoven fabric, an inherently stiff molded part is obtained in which—if it is mounted as an acoustic absorber in front of a reflective plane, such as the sheet-metal wall of an automobile—a not insignificant air gap is formed between the absorber and the reflective plane due to the spacer or spacers, the additional air volume that is created in this way acting as an integral part of the structure of the absorber. This makes it possible to obtain a molded part from meltblown nonwoven fabric with an outstanding absorbent effect at a substantially reduced material cost. By virtue of the volume of air that is enclosed between absorber and wall, a substantial improvement of the low-frequency behavior of the construction is achieved that can otherwise only be achieved by correspondingly thick and hence also heavy and expensive materials. In another embodiment of the invention, the volume of air between the absorber and wall described above can also be created by a structure of the wall in the case of a flat absorber or a structure of the wall and of the absorber, the inherent stiffness of the absorber being necessary for the permanent formation of the volume of air.

As stated, the meltblown nonwoven fabric to be subjected to annealing can be manufactured by means of any of the known meltblown processes, such as those described in U.S. Pat. Nos. 4,118,531, 4,375,446, 4,380,570, or DE 17 85 712 C3, for example. As a basic principle, nonwoven fabric is manufactured using a meltblown process by applying flowing air to the outside of a polymer melt that is extruded through a die and drawing said polymer melt before the filaments that are formed in this way are placed onto a carrier and cooled. The carrier is preferably a double suction drum.

As stated, the degree of crystallinity of the meltblown nonwoven fabric is increased by the annealing. Preferably, the filaments of the annealed meltblown nonwoven fabric have a degree of crystallinity at least in portions and preferably over the entire surface of from 20 to 80%, more preferably from 30 to 75%, especially preferably from 40 to 75%, and most preferably from 50 to 70%. Analogously, if the meltblown nonwoven fabric is annealed only in portions, the annealed regions of the annealed meltblown nonwoven fabric preferably have a degree of crystallinity of from 20 to 80%, more preferably from 30 to 75%, especially preferably from 40 to 75%, and most preferably from 50 to 70%. According to the invention, the meltblown nonwoven fabric has, at least in portions and preferably over its entire surface, a compression hardness (compressive stress) at 60% compression of at least 2 kPa as measured according to DIN EN ISO 3386. Especially preferably, the meltblown nonwoven fabric has a compression hardness (compressive stress) at least in portions and preferably over its entire surface at 60% compression of at least 4 kPa, more preferably of at least 8 kPa, even more preferably of at least 10 kPa, even more especially preferably of at least 12 kPa, yet more especially preferably of at least 15 kPa, very especially preferably of at least 20 kPa, and most preferably of at least 30 kPa as measured according to DIN EN ISO 3386. In departure from the abovementioned standard, the compression hardness at 60% compression is to be understood as the required compressive stress under which a material sample undergoes a change in thickness by 60% of its initial thickness. Furthermore, the preload for determining the initial thickness of the material is reduced to 0.014 kPa in order to take the very low compression hardness of the unannealed material into account. In case of degrees of compression or other test conditions that deviate therefrom, deviating compressive stresses with nonlinear correlations to the cited values can arise.

In order to shorten the annealing time, it is proposed in a development of the inventive idea that the annealing temperature be increased in a continuous or stepwise manner during annealing, preferably even beyond the melting temperature of the unannealed filaments of the meltblown nonwoven fabric, but on the proviso that the annealing temperature be always at least 0.1° C. below the current melting temperature of the filaments of the meltblown nonwoven fabric existing at this point in time.

Overall, the present invention makes it possible to increase the degree of crystallinity of the filaments of meltblown nonwoven fabrics in portions or over their entire surface and thus the stiffness of meltblown nonwoven fabrics in portions or over their entire surface. In particular, the present invention can be used to anneal the entire surface of the meltblown nonwoven and thus to increase the degree of crystallinity over the entire surface of the meltblown nonwoven fabric. This enables rigid, pressure-stable two-dimensional components to be produced. As an alternative to this, the shaped meltblown nonwoven fabric can also be annealed only in portions, thereby increasing the degree of crystallinity in the meltblown nonwoven fabric only over a portion of its surface, for example in order to increase the stiffness only in component-specific regions or in the continuous grid of the component. For example, it is possible for only the edge regions of the component made of the meltblown nonwoven fabric to be annealed in order to make the edge regions of the component stiffer, for example in order to increase the stackability of the component made of the meltblown nonwoven fabric. Alternatively, the annealing of the meltblown nonwoven fabric enables a component to be shaped and the degree of crystallinity therein increased over its entire surface in order to manufacture inherently rigid three-dimensional components. On the other hand, it is also possible to deform the meltblown nonwoven fabric only in portions of its surface and to increase the degree of crystallinity only in these portions of the surface, for example in order to thereby form one or more spacers or different local functional geometry in the meltblown nonwoven fabric. In all of the abovementioned possible applications, locally compressed or consolidated regions can expand the functionality, particularly in order to form contact surfaces at attachment points, for example.

Another object of the present invention is an annealed meltblown nonwoven fabric whose filaments have a degree of crystallinity at least in portions and preferably over their entire surface of from 20 to 80%, preferably from 30 to 75%, especially preferably from 40 to 75%, and most preferably from 50 to 70%.

The present invention further relates to a meltblown nonwoven fabric that has, at least in portions and preferably over its entire surface, a compression hardness at 60% compression of at least 2 kPa as measured according to DIN EN ISO 3386. Preferably, the meltblown nonwoven fabric has a compression hardness at 60% compression of at least 4 kPa, especially preferably of at least 6 kPa, more preferably of at least 8 kPa, even more preferably of at least 10 kPa, even more preferably of at least 12 kPa, yet more preferably of at least 15 kPa, very especially preferably of at least 20 kPa, and most preferably of at least 30 kPa.

Another object of the object of the present invention is a method for manufacturing an annealed meltblown nonwoven fabric with a weight per unit area of from 100 to 600 g/m² and with a density of from 5 to 50 kg/m³, comprising the following steps:

-   a) manufacturing a meltblown nonwoven fabric, preferably by applying     flowing air to the outside of a polymer melt that is extruded     through a die and drawing said polymer melt before the filaments     that are formed in this way are placed onto a carrier, which is     preferably a dual suction drum, and cooled, and -   b) annealing at least a portion of the meltblown nonwoven fabric     prepared in step a) at a temperature that lies between the glass     transition temperature and 0.1° C. below the melting temperature of     the filaments of the meltblown nonwoven fabric.

The method steps described above as being preferred for the meltblown nonwoven fabric according to the invention also apply to the method according to the invention.

Accordingly, the meltblown nonwoven fabric is especially preferably annealed in step b) for 2 minutes to 2 hours at a temperature that lies between 20° C. below the melting temperature and 1° C. below the melting temperature of the filaments of the meltblown nonwoven fabric. Hereinafter, the present invention will be described below with reference to the clarifying but non-limiting drawing. In the drawing:

FIG. 1 shows a schematic of a furnace for manufacturing an annealed meltblown nonwoven fabric according to one exemplary embodiment of the present invention.

FIG. 2 shows a schematic of a mold for the simultaneous shaping and annealing of a meltblown nonwoven fabric according to another exemplary embodiment of the present invention.

FIG. 3 shows a comparison of the compression hardness of an annealed meltblown nonwoven fabric according to another exemplary embodiment of the present invention to the compression hardness of an unannealed meltblown nonwoven fabric according to the prior art.

FIG. 4 shows the results of the measurement of sound absorption of annealed meltblown nonwoven fabric prepared in example 1 according to the present invention (curve A) in comparison to the unannealed meltblown nonwoven fabric manufactured in the comparative example (curve B).

FIG. 5 shows the results of the measurement of the absorption coefficient of the annealed meltblown nonwoven fabric prepared in example 1 mounted directly against a body panel (curve A), mounted on a body panel at a distance of 10 mm (curve B), and mounted on a body panel at a distance of 40 mm (curve C).

FIG. 1 shows a schematic of a belt furnace 10 for manufacturing an annealed meltblown nonwoven fabric according to one exemplary embodiment of the present invention. The furnace 10 comprises air-permeable belts 14, 14′ that are guided and driven on rollers 12 over which the meltblown nonwoven fabric 15 is guided into and through the furnace 10. A first blast box 16, a suction box 18, and a second blast box 16′ are arranged in the furnace 10 above and below the two belts 14, 14′ in this sequence as seen from right to left in the direction of conveyance. During operation of the furnace 10, the meltblown nonwoven fabric 15 is fed from right to left on the lower belt 14 through the furnace 10. As the meltblown nonwoven fabric 15 passes through the blast boxes 16, 16′, hot air is flowed into and through it in order to elevate the filaments of the meltblown nonwoven fabric 15 to the desired annealing temperature. Air flowing through the meltblown nonwoven fabric 15 is extracted in the vicinity of the suction box 18 in order to ensure that the meltblown nonwoven fabric 15 is reliably flowed through by the hot air and the meltblown nonwoven fabric 15 also does not collapse but rather retains its volume.

FIG. 2 shows a schematic of a mold 20 for the simultaneous shaping and annealing of a meltblown nonwoven fabric 15 according to another exemplary embodiment of the present invention. The meltblown nonwoven fabric 15 is maintained in the desired shape from both sides by appropriately shaped screens 22, 22′ of which the mold 20 is composed and heated to the desired temperature for annealing through passage of air around or through. The nonwoven mat manufactured in this way retains the impressed shape and is dimensionally stable.

FIG. 3 shows the compression hardness of an annealed meltblown nonwoven fabric at 60% compression with a weight per unit area of about 300 g/m2 and a density of about 15 kg/m3 according to another exemplary embodiment of the present invention (upper curve) and the compression hardness of an unannealed meltblown nonwoven fabric with the same weight per unit area and the same density according to the prior art (lower curve) in a comparison. The compression hardness is shown as compression in % versus the compressive stress in kPa. As can be seen from FIG. 3, a compressive stress of about 12 kPa is required in the annealed meltblown nonwoven fabric (upper curve) in order to achieve a compression of 60%, whereas the same compression is already achieved in the unannealed meltblown nonwoven fabric according to the prior art (lower curve) at about 1.5 kPa. This demonstrates strikingly that the compression hardness can be increased dramatically in a voluminous meltblown nonwoven fabric through annealing.

The present invention will be described below with reference to clarifying but non-limiting examples.

EXAMPLE 1

A meltblown nonwoven fabric with a weight per unit area of 300 g/m2 and with a density of 15 kg/m3 was manufactured from filaments composed of isotactic polypropylene with a mean filament fineness of 5 μm using the meltblown process described in U.S. Pat. No. 4,375,446. This meltblown nonwoven fabric was then annealed in a circulating-air furnace for 10 minutes at 158° C. As a result of the placement of the cold nonwoven fabric and the opening of the furnace door, the initial temperature was below the melting point of the filaments of the unannealed nonwoven fabric. Through the immediately incipient crystallization with associated increase in the melting point of the filaments, it was possible to continue annealing for the rest of the 10 minutes at 158° C., i.e., above the melting temperature of the unannealed filaments but below the current melting temperature of the filaments at that point in time, thereby shortening the annealing time in comparison to annealing at a lower temperature. The compression hardness at 40% compression and the compression hardness at 60% compression of the annealed meltblown nonwoven were then measured according to DIN EN ISO 3386. The results are summarized in table 1 below and show that the annealing according to the invention results in a drastic increase in the compression hardness.

In addition, the sound absorption coefficient of the annealed meltblown nonwoven fabric was measured according to DIN EN ISO 10534 as a function of the thickness-normalized frequency. The results are shown in FIG. 4 in curve A in comparison to the values that were achieved with the unannealed meltblown nonwoven fabric that was manufactured in the comparative example (curve B). The unit of the abscissa is the measurement frequency×absorber thickness/15 mm. The comparison of the results shows that the annealing according to the invention has no negative influences on the sound absorption characteristics of the nonwoven fabric.

A portion of the annealed meltblown nonwoven fabric was mounted directly against a vehicle body panel, whereas another portion of the annealed meltblown nonwoven fabric was mounted on a vehicle body panel at a distance of 10 mm and another portion of the annealed meltblown nonwoven fabric was mounted on a vehicle body panel at a distance of 40 mm. The absorption coefficient was then determined for the three constructions as a function of frequency. The results are shown in FIG. 5; curve A shows the values for the meltblown nonwoven fabric that was mounted directly against the vehicle body panel, curve B shows the values for the meltblown nonwoven fabric that was mounted on the vehicle body panel at a distance of 10 mm, and curve C howl the values for the meltblown nonwoven fabric that was mounted on the vehicle body panel at a distance of 40 mm. A comparison of the values obtained shows that the volume of air trapped between nonwoven fabric and body panel results in a clear improvement particularly in the low-frequency absorption characteristics of the construction that can otherwise only be achieved by means of correspondingly thick and hence also heavy and expensive materials.

EXAMPLE 2

An annealed meltblown nonwoven fabric was manufactured according to the procedure described in example 1, except that the annealing was carried out at 155° C. for 10 minutes.

EXAMPLE 3

An annealed meltblown nonwoven fabric was manufactured according to the procedure described in example 1, except that the annealing was carried out at 155° C. for 25 minutes.

COMPARATIVE EXAMPLE

An unannealed meltblown nonwoven fabric was manufactured according to the procedure described in example 1, except that the annealing described in example 1 was not carried out.

TABLE 1 Compression Compression Annealing Annealing hardness hardness temperature time factor at 40% factor at 60% Example (°C) (min.) compression compression 1 158 10 18.5 14 2 155 10 9.5 7 3 155 25 12 9 Comparative — — 1 1 example 1 Compressive hardness factor: Ratio of the compression hardness of the annealed nonwoven fabric of the example divided by the compression hardness of the unannealed nonwoven fabric of the comparative example

A comparison of the results shows that the subsequent annealing of the meltblown nonwoven fabric results in a drastic increase in the compression hardness of the meltblown nonwoven fabric.

LIST OF REFERENCE SYMBOLS

-   10 (belt) furnace -   12 rollers -   14, 14′ air-permeable belt -   15 meltblown nonwoven fabric -   16, 16′ blast box -   18 suction box -   20 mold -   22, 22′ screen 

1-15. (canceled)
 16. An annealed meltblown nonwoven fabric, obtainable by means of a method in which at least a portion of the meltblown nonwoven fabric (15) is subsequently annealed at a temperature between the glass transition temperature and 0.1° C. below the current melting temperature of the filaments of the meltblown nonwoven fabric (15), wherein the meltblown nonwoven fabric (15) is composed of a polyolefin, and the meltblown nonwoven fabric (15) has a weight per unit area of from 100 to 600 g/m², a density of from 5 to 50 kg/m³, and a compression hardness at 60% compression of at least 2 kPa as measured according to DIN EN ISO
 3386. 17. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) is annealed at a temperature between 20° C. and 1° C. below the current melting temperature of the filaments of the meltblown nonwoven fabric (15).
 18. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) is annealed at the temperature for 1 minute to 10 days.
 19. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) is annealed through exposure to hot air and/or superheated steam.
 20. The meltblown nonwoven fabric as set forth in claim 19, wherein the meltblown nonwoven fabric (15) is annealed in a furnace (10) having at least one blast box (16, 16′) and at least one suction box (18), the at least one blast box (16, 16′) being arranged such that the hot air can be blown into the meltblown nonwoven fabric (15), and the at least one suction box (18) being arranged such that air flowing through the meltblown nonwoven fabric (15) can be extracted.
 21. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) has a weight per unit area of from 100 to 400 g/m².
 22. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) is a voluminous meltblown nonwoven fabric (15) having a density of from 7 to 40 kg/m³.
 23. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) is composed of filaments that are made of a polypropylene and/or polyethylene.
 24. The meltblown nonwoven fabric as set forth in claim 16, wherein the thickness of the meltblown nonwoven fabric (15) is from 6 to 50 mm.
 25. The meltblown nonwoven fabric as set forth in claim 16, wherein i) the meltblown nonwoven fabric (15) is annealed in a mold (20) for the purpose of reshaping it during annealing, the mold (20) being embodied at least partially as a screen (22, 22′), so that the meltblown nonwoven fabric (15) can be flowed through and/or flowed around by hot air and/or with superheated steam during annealing, and/or ii) the meltblown nonwoven fabric (15) is transferred to a mold (20) after heating for the purpose of reshaping it, in which case the meltblown nonwoven fabric (15) is cooled in the mold in order to conclude the annealing process.
 26. The meltblown nonwoven fabric as set forth in claim 16, wherein at least one spacer is provided in the meltblown nonwoven fabric (15) that is arranged in the direction of thickness of the meltblown nonwoven fabric (15) and, as a result of permanent molding, has a length that is greater than the length of the meltblown nonwoven fabric (15).
 27. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) that is subsequently annealed was manufactured by applying flowing air to the outside of a polymer melt that is extruded through a die and drawing said polymer melt before the filaments that are formed in this way are placed onto a carrier and cooled.
 28. The meltblown nonwoven fabric as set forth in claim 16, wherein the meltblown nonwoven fabric (15) has a compression hardness at 60% compression of at least 4 kPa as measured according to DIN EN ISO
 3386. 29. The meltblown nonwoven fabric as set forth in claim 16, wherein the annealing temperature is increased in a continuous or stepwise manner during annealing, but on the proviso that the annealing temperature be always at least 0.1° C. below the current melting temperature of the filaments of the meltblown nonwoven fabric existing at this point in time.
 30. A method for manufacturing an annealed meltblown nonwoven fabric having a weight per unit area of from 100 to 600 g/m² and having a density of from 5 to 50 kg/m³, comprising the following steps: a) manufacturing a meltblown nonwoven fabric (15) and b) annealing at least a portion of the meltblown nonwoven fabric prepared in step a) at a temperature between the glass transition temperature and 0.1° C. below the melting temperature of the filaments of the meltblown nonwoven fabric. 